1. Field of the Invention
The present invention relates to an image bearing member, and an image forming apparatus and a process cartridge using the image bearing member. The image forming apparatus and the process cartridge are widely applicable to photocopiers, facsimile machines, laser printers, direct digital platenmakers, etc.
2. Discussion of the Background
In recent years, image forming apparatuses such as laser printers, digital photocopiers that employ an electrophotographic system have been improved with regard to image quality and reliability thereof and are now widely diffused.
Such image forming apparatuses use an image bearing member, the surface of which is charged and irradiated with light to form latent electrostatic images thereon, followed by development to obtain a visual image. This image bearing member may be a photoreceptor or photoconductor.
The image bearing members are classifiable generally into inorganic photoconductors and organic photoconductors. Organic photoconductors have become widely diffused due to the advantages they provide in terms of cost, productivity, variety of choices of usable materials, impact on the environment, etc. Such organic photoconductors are formed of a photosensitive layer mainly containing a photosensitive material, and can be classified into two main types: a single-layer type in which a single layer assumes both a charge generation function and a charge transport function, and a laminate type having a charge generation layer assuming a charge generation function and a separate charge transport layer assuming a charge transport function.
The mechanism of forming latent electrostatic images on an image bearing member of a functionally separated laminate type involves irradiating a uniformly charged image bearing member with light, which light then passes through the charge transport layer and is absorbed in charge generation material in the charge generation layer, resulting in generation of charge, actually a pair of charges. One of the pair is infused into the charge transport layer at the interface between the charge generation layer and the charge transport layer, and moved through the charge transport layer by an electric field to the surface of the image bearing member where it then neutralizes the surface charge provided by charging, resulting in formation of a latent electrostatic image. Organic photoconductors having such a laminate structure are advantageous in terms of stability of electrostatic characteristics and durability, so that organic photoconductors are currently the main type of image bearing member in use today.
Recently, with rapid advances in full-colorization and high-speed performance, demand for image forming apparatuses using such organic photoconductors has come from a wide range of fields, from the typical business office to the fields of SOHO (small office home office) or quick printing.
In the quick printing field in particular, printing volume has increased dramatically, together with demand for reliably high-quality images. Therefore, organic photoconductors having high durability and reliability are indispensable for meeting such requirements.
However, for an organic photoconductor to have a long working life, abrasion resistance thereof should be improved. Accordingly, a number of approaches have been tried.
For example, Japanese patent application publication nos. (hereinafter referred to as JP-A) JP-S56-48637-A describes using a curable binder resin for the surface layer; JP-S64-1728-A describes using a charge transport polymer; JP-H04-281461-A describes using an inorganic filler in the surface layer; and JP-2005-99688-A describes dispersing filler particulates in the surface layer.
However, these technologies are not sufficiently successful in the quick printing field because of problems related to abrasion resistance. Moreover, even if abrasion resistance is not a fatal problem, other factors such as filming, attachment of foreign matter, and cleaning performance cause problems. Therefore, the photoreceptors need replacing in a relatively short time.
This is because when using a photoreceptor having improved abrasion resistance, the surface of the photoreceptor is hardly refaced. Therefore, the surface of the photoreceptor is contaminated, resulting in production of abnormal images.
There are known technologies to prevent the filming or the attachment of foreign objects to the surface of a photoreceptor. For example, JP-H02-139566-A and JP-H02-150850-A describe roughening the surface of a photoreceptor. Also, JP-H02-144551-A describes adding a lubricant to the surface of a photoreceptor and JP-2002-287567-A and JP-2007-79244-A describe coating the surface of a photoreceptor with a lubricant.
These technologies are effective to improve the lubricant property of the surface of the photoreceptor. However, coating the surface of a photoreceptor with lubricant has a problem with sustainability of the effect.
That is, it is difficult to limit the contamination of a photoreceptor caused by repeated image formation processes such as charging, development, and cleaning for an extended period of time using only the lubricant contained in the photoreceptor from the beginning.
On the other hand, since the lubricant is securely applied to the surface of a photoreceptor by coating the surface of the photoreceptor with lubricant, the lubricating effect is sustained even when the surface of the photoreceptor is contaminated. Consequently, the operation of the cleaning blade is stabilized, which leads to a reduction in both poor cleaning performance and attachment of foreign matter.
Therefore, in the case of the quick printing field, which requires a highly durable, reliable photoreceptor, coating the surface of the photoreceptor with lubricant is extremely effective.
However, there are other problems to be solved to obtain a photoreceptor having improved durability.
Due to repetitive charging and irradiation of the photoreceptor, electrostatic stability deteriorates sharply, with a rise in the voltage of the irradiated portion of the photoreceptor and a decrease in the voltage of the dark portion extremely. This causes image density to fluctuate, and image blur occurs due to oxidized gas produced by a charger during charging.
To reduce image blur caused by oxidized gas, and improve gas resistance, JP-S59-136744-A and Japanese patent no. 4194776 (JP-4194776-B) describe adding various kinds of anti-oxidants to the photosensitive layer or the surface layer of the image bearing member.
However, known anti-oxidants have little effect in preventing image blur, and have a side effect of increasing the voltage at the irradiated portion. Therefore, to reduce image blur, a large quantity of anti-oxidant must be added. Consequently, the voltage at the irradiated portion increases sharply, which degrades image density stability.
JP-2000-292959-A describes reducing the voltage at the irradiated portion such that the ionization potential of the charge transport material contained in the protection layer is equal to or less than that of the charge transport material contained in the charge transport layer formed under the protection layer. Charge transport material having a small ionization potential is suitable for reducing the voltage at the irradiated portion in terms of the infusion barrier of the charge at the interface.
However, charge transport materials having a small ionization potential are generally inferior in stability and particularly susceptible to the effects of oxidized gas, resulting in occurrence of severe image blur. Therefore, just using a transport material having a small ionization potential is not necessarily a solution to the problem.
In addition, JP-2006-138956-A describes a photoreceptor having a cross-linked charge transport layer in which the cross-linked charge transport layer has the same charge transport material having a small molecular weight as the material contained in the charge transport layer provided below the cross-linked charge transport layer, and the density of the charge transport material gradually decreases from the side of the charge transport layer. Therefore, the abrasion resistance of the photoreceptor is improved and the residual voltage can be lowered. However, occurrence of image blur caused by repetitive use for an extended period of time is not prevented.
As an example of adding an anti-oxidant to the surface layer, JP-2008-70553-A describes a photoreceptor in which the surface layer contains a compound having both a hindered phenol structure and a hindered amine structure, and the density of the compound is different at the surface and in the interior of the photoreceptor. Although successful in reducing the occurrence of negative residual images, no description is given of handling image blur.
On the other hand, Japanese patent nos. 3963445 and 4112444, and JP-2007-233425-A and JP-2007-279678, describe adding a compound having an alkyl amino group to the photosensitive layer or the surface layer to serve the same purpose. These technologies are preferable because occurrence of image blur is reduced and the rise in the voltage at the irradiated portion has relatively less impact than in the case of the anti-oxidant.
However, although sufficient to solve the problems related to the durability of the photoreceptor that used to be required, these approaches fail to provide the durability required by the field of quick printing.
The image blur ascribable to the repetitive use of the photoreceptor is thought to occur by transmutation of a composition such as the charge transport material forming the photoreceptor when the photoreceptor is exposed to the oxidized gas produced by the charging device. A compound having an alkyl amino group is considered to be suitable to reduce this transmutation, but the amount of compound required must increase as the frequency of use or the period of use of the photoreceptor increases accordingly. However, if the amount increases, the problem that the voltage at the irradiated portion increases over time inevitably arises.
This problem of fluctuation in the voltage at the irradiated portion in the image forming apparatus for use in the quick printing field is more serious when printing is resumed after one job than when images are output for a relatively long period of time. Hereinafter, the latter is referred to as intraday fluctuation of the voltage at the irradiated portion, and, the former, intra-job fluctuation of the voltage at the irradiated portion.
In the case of the intraday fluctuation of the voltage at the irradiated portion, no adverse impact is noticeable, and the voltage can be corrected in the device so that this fluctuation does not cause a great problem. However, when the intra-job fluctuation of the voltage at the irradiated portion is significant, its impact cannot be ignored or go unnoticed. If the voltage fluctuates per output of several tens of images or several images, the correction of the voltage is difficult, which leads to a serious problem.
Moreover, often the same image pattern is output in a large quantity in one job particularly in the quick printing field. If the intra-job fluctuation at the irradiated portion is significant in such a case, the image density changes, resulting in deterioration of image consistency. This is not so noticeable when the image pattern is mainly formed of text. By contrast, when outputting, for example, a full color image, not only the image density but also the color tone change, which creates an extremely serious problem. That is, it is necessary to reduce the voltage at the irradiated portion of the photoreceptor, the fluctuation of the voltage in one day during repetitive printing for a long period of time, and the fluctuation of the voltage in one job.
Furthermore, the photoreceptor is required to be free from the occurrence of image blur or decrease in resolution, reduce the occurrence of filming, attachment of foreign matter, and poor cleaning performance, and have an abrasion resistance or durability good enough to lead a working life to the full.
As described above, the image forming apparatus for use in the quick printing field is required to have an image bearing member (photoreceptor) with good voltage stability over repetitive use, and which reduces production of blurred images as well as abnormal images stemming from filming, poor cleaning performance, etc.